Electric switch



Sept. 7, 1943. J. w. SEAMAN ELECTRIC SWITCH Filed May 21, 1942 2Sheets-Sheet 1 Insulation Insulation Insulation .m m y f e owwfi b r m)o e am vb W mw fi y y b p 7. 9 .1. w. SEAMAN 2,329,003

ELECTRIC SWITCH Filed May 21, 1942 2 Sheets-Sheet 2 InsulationInsulation 33 :35 34 v n-5i In; r I /4 E:E L! Y "2325M gig? a2 0- g 2s\1/ x I 22 25K ,6 28 2/ f [/16 O l g (A) 7; ,9 5 Insulatron Q,

5 Insul/ation LF LF 0 0 0 IYWVQYWQOY: Joseph W. Sea-man,

M a Add by His Attorney Patented Sept. 7, 1943 nmc'rmc swrrcn Joseph W.Seaman, Thornton, DeL, minor to General Electric Company, a. corporationof New York Application May 21, 1942, Serial No. 443,878

14 Claims.

My invention relates to improvements in electric switches and moreparticularly high-speed circuit breakers, and especially circuitbreakers which have a relatively high current-carrying capacity and yetoperate to interrupt a direct current or rectified alternating currentcircuit whose current is increasing at a high rate before the magnitudeof the current can attain dangercus values.

Probably the greatest trouble experienced in the operation of mercuryarc rectifiers arises from abnormal operating conditions such as anarc-back occasioned by reverse current flow from cathode to. anode or bythe striking of an arc discharge between anodes. The reversal of currentflow establishes a short circuit which is fed not only by the otherrectifying paths of the particular rectifier, but also by all therectiflers operating in parallel with this rectifier. The rate ofincrease of current in this short circuit path is so high, particularlyin large installations,that currents dangerous to equipment andpersonnel and destructive of service continuity are quickly reached.There have been observed rates of increase of current of over sixmillion amperes a second for a single rectifier and over eleven millionamperes a second where several are operated in parallel. It is,therefore, imperative to interrupt this short circuit current veryquickly if the current at interruption is not to exceed a safe value andinterruption of service is to be prevented. In short, the problem is tointerrupt the circuit of the faulty anode so quickly as to limit theresulting current to a safe value.

Even in normal parallel operation of rectifiers the pole units of theinterrupting means must be able to carry relatively high currentsbecause the poles of the circuit breaker carrying currents in the normalor forward direction during an arc-back are subjected to abnormallyhigh'current flow. The magnetic effect of such relatively high currentsin the passage through the conducting path of the usual interruptingmeans tends to decrease the conductivity of the current path by reducingthe pressure at the contacts. This can result in dangerou heating andburning of the contacts and may thus jeopardize the safety of theequipment. This is particularly true in circuit breakers embodying pointor line contact action since high unit contact pressure is essential tohigh conductivity; On the.

should expedite rather than delay the circuit in terruption. Obviously,any circuit'interrupting device which dependably fulfills therequirement of high-speed interruption and yet provides highconductivity at normal currents must also be econonically feasible inorder to be accepted by the ar One object of my invention is to providean improved circuit breaker, the inertia of the movable contact parts ofwhich is reduced to a minimum consistent with the requiredcurrent-carrying and interrupting capacities. Another object of myinvention is to provide an improved circuit breaker structure in whichflexibleconductors such as pigtails and the like are unnecessary. Stillanother object of my invention is to provide an improved circuit breakerwhose stationary and movable conducting parts are so arranged as tofacilitate a quick opening operation. A further object of my invention ito provide an improved circuit breaker, the stationary and movableconducting parts of which are so arranged as to utilize the so-calledmagnetic loop expansion effect to increase the contact pressure andthereby to obtain better conductivity not only under normal circuitconditions but also in case of high current flow in a direction forwhich circuit opening is not desired and to expedite the circuit openingoperation-under such abnormal circuit conditions as require it. Theseand other objects of my invention will appear in more detailhereinafter.

In accordance with my invention, I reduce the weights of the movingcontact elements in the circuit breaker and the frictional losses at thecontact engaging surfaces to a minimumin order to obtain the quickestopening operation consistent with the necessary current-carrying andinterrupting capacities. Also in accordance with my invention, Ieliminate the use of flexible conductors, such as so-called pigtails andthe like. Further in accordance with my invention,I so arrange thestationary and movable conducting elements of the circuit breakerrelatively to each other and their positions in the current conductingloop through the circuit breaker that the magnetic loop expansion effectincreases the pressure at the cooperating contact surfaces under normalconditions and accelerates th separation of the contacts when theyshould no longer be maintained in the circuit closing position, as incase of tripping .during abnormal circuit conditions.

' My'invention will be better understood from the following descriptionwhen considered in connection with the accompanying two sheets ofdrawings, and its scope will be pointed out in the appended claims.

In the accompanying drawings, Fig. 1 i a ver tical side elevation,partly broken away and partly in section, of a circuit breaker embodyingmy invention; Fig. 2 is a partial elevation similar to Fig. 1 with thecontacts in the circuit opening position; Fig. 3 is a fragmentary sideelevation on an enlarged scale illustrating in detail the mounting ofone of the contacts of the circuit breaker; and Figs. 4 and 5 areelevations of insulating piece used in connection with the mountingshown in-Fig. 3.

In the illustrated embodiment of my invention,.I have shown an electricswitch having a generally U-shaped current conducting loop comprisingtwo spaced current conducting studs l and II. These are mounted on asuitable supporting structure such as a panel [2 of suitable insulatingmaterial. The lower stud l9 includes a terminal 13 and the upper stud llincludes a terminal [4. In accordance with my invention, I provide amovable contact l fulcrumed for movement about a given axis and one ormore relatively stationary contacts l6 and I! fulcrumed for movementeach about its own independent axis and respectively engageable by thecontact between its axis and the respective axes of the contacts [6 andIT. The construction and ar- 7 rangement of parts is such that themagnetic loop expansion effect consequent upon current conducted by thecontacts tends to increase the pressures between their engaging portionsand between the contacts and their supports and, also,

to accelerate the circuit opening movement of the movable contact 15when it is free to move to the circuit opening position.

In order to obtain the desired contact action, the contact 15 is, inaccordance with my invention, mounted for rocking action about astationary contact surface I8 which faces toward the inside of the loopso that the loop expansion effect forces the cooperating contact surface[9 of the contact 15 against the contact surface 18. This contactsurface i8 is on a contact block which forms a part of the stud [0. Asshown, th contact surface i 8 is curved and convexly so toward theinside of the loop. The surface 18 may be a cylindrical surface, asshown, and the cooperating contact surface [9 a plane surface, as shown.With this arrangement, I provide for the movable contact 15, inaccordance with my invention, so as to minimize the scuffing and wearbetween the contact surfaces 18 and IS, a supporting means such as acradle 2!, which is pivotally supported for movement about an axis lyingin the cylindrical contact surface 18. This axis is substantially thecentral element of the cylindrical surface IS. The contact I5 ispivotally supported on a pin 22 mounted in the upper end of the cradle2|. This cradle actually comprises two spaced side portions, one on eachside of the contact I5. The one on the near side is omitted in thedrawings for the sake of clearness in illustration.

In order to obtain multipl point contact or the equivalent thereof so asto conduct even higher transient currents, the bridging contact l5 mayconsist of a plurality of relatively thin, similar contacts, asdisclosed, for example, in Letters Patent of the United States2,302,394, issued November 17, 1942.

The stud I I comprises a contact block 23 having a substantiallycylindrical recess 24 in which the upper end of the contact l6 fitssomewhat loosely. Actually, I prefer to make the contact by suitableinsulation 35.

16 a plurality of separately spaced parallel contact elements or fingersI5 and I 6', as shown more clearly in Figs. 2 and 3, with theintermediate contactfinger l5 so shaped and biased as to provide aspaced sequence in engagement and separation by the contact [5 sincethis provides a better contact action. As shown in Fig. 3, the mountingfor each contact element 15 and I6 includes a headed screw 25, which issupported in an opening in the stud II and screwed into the contactblock 23. The headed screw 25 carries a contact biasing or backingspring 26. Inorder to maintain a sharp current loop at the contactrecess 24, thehead of the screw 25 is centered in the opening of thestud II by suitable insulating means such as a bushing 2! to prevent thepassage of current from th block 23 through the screw 25 and the spring25 to the stud II. The bias of the backing spring 26 holds the upper endof the associated contact element I6 or IE forward against the face ofthe recess 24 in line to line contact in the arrangement shown. For

better contact action, the upper end of the contact elements 16, I8 andthe surface of the recess 24 against which they bear may be silverfaced. Likewise, the lower end of each of the contact elements l6, I6 ispreferably provided with a contact piece 28 of suitable contact materialsuch, for example, as silver. These silver contact pieces 28 engage asilver contact piece 29 mounted on the movable contact 15. In order toprevent conduction except through the line to line engagement of thefingers 16, I6 in the recess 24, I further provide an insulating strip30 between the contact fingers !6, I5 and the overhanging portion 31 ofthe contact block 23. The contact fingers l6, iii are spaced slightly byinsulating separators 32 in order that each finger may be free to moveindependently of the others under the magnetic attracting forces betweenthem due to the currents flowing through them in the same direction.This arrangement permits the Contact fingers l6, Hi to come to a betterbearing in the contact recess 24, and similarly the contact pieces 28come to a better bearing against the contact piece 29.

In accordance with my invention, the stationary or arcing contact I"! ismounted in a substantially cylindrical recess 33 in an arcing contactblock 34, which is separated from the contact block 23 Like the contactfingers l6, 16', the contact I! is biased forward to engage the arcingcontact block 34 in line to line contact. For this purpose, I provideone or more backing springs 36, which are mounted in holes in the stud Hand the contact block 23. The backing springs 36 do not engage thecontact H directly but are spaced therefrom by insulating rods 31. Thelower end of the contact I! and the upper end of the movable contact [5are provided with suitable contact engaging pieces 38.

For arc extinguishing purposes, the arcing contact block 34 is providedwith an arcing horn extension 39 and is also connected to the stud llthrough a blowout coil 40. For carrying the end of the arc off from thecontact portion 38 of the movable contact 15, I provide a blowout coil4| having a suitable core 42. This coil is connected between an arcinghorn 43 and the lower stud ID of the circuit breaker through a conductor44. This are extinguishing structure further includes pole pieces 40',only one of whichis shown, for the core of the coil 4|! and twoenclosing barriers 45, only one of which appears in the drawings.

In order to move the cradle 2| into and maintaln it in the circuitclosing position and yet have it quickl freed for high-speedinterruption, suitable operating means is provided. The particular meansillustrated comprises a movableand are rollers, as shown. The abutment45 v is mounted on a pivot 52 extending between the sides 48 of thecircuit breaker frame. In order to obtain the counterclockwise movementof the cradle 2i necessary to bring the contact I5 into the normalcurrent conducting position, the abutment 48 is provided with a suitablyshaped ,cam face 53.

asaaooe contact surfaces l8 and I8 at all times and also returning theoperating member 54 and the parts connected thereto into position inpreparation For actuating the roller 50, there is provided I anoperating member 54, which is mounted on the pivot 52 and is connectedto the roller 50 through one or more operating links 55. These links arepivoted to the operating member 54 at 56 and to the roller '50 at 51.The operating member 54 comprises two side pieces 54 and a crossconnection 58, which in the closed position of the circuit breaker abutsthe frame 48 at 59, as shown in Fig. 1. circuit breaker in the closedposition as long as In order to maintain the,

the abutment 46 is maintained in the latched v position, the parts areso proportioned and arranged that in the closed position of the circuitbreaker the direction of pressure on the roller 55 tends to force thisroller to the right on the cam surface 53.

for reclosing the circuit breaker, there is provided suitable energystoring means such as a biasing spring 10. In order that this spring maybe suitably stressed during the closing movement of the circuit breaker,it is connected between the pivot 55 and a rod 1| fixed in the contactl5, which is provided with a hollow portion for receiving the end of thespring.

Since mercury arc rectiflers usually have six anode circuits andsometimes more, the operating mechanism illustrated provides anarrangement whereby a plurality of single-pole circuit breakers in theopen circuit position can be closed simultaneously, and also any one ormore poles can open and can be closed without interfering with thepoles. that are closed whereby to avoid unnecessary interruptions insound anode circuits. For this purpose, each pole comprises an actuatinglever 12. These are interconnected by a common actuating meanscomprising a crossbar 18. As shown, the actuating lever 12 is U- shapedwith across piece 14 having a projection 15 arranged to abut the crosspiece 58 of the operating crank 54. In order to insure that each polewill be positively closed, theprojection 15 is adjustably positioned soas to take care of manufacturing tolerances and variations in lost.

motion whereby to insure the desired closing action of any one or morepoles even though the others are closed. The projection 15 is so ad-'justed that a predetermined clockwise movement of the actuating lever14 will impart sufilcient clockwise movement to the operating crank 54to. effect the holding bias of the rollers 50 and 5i.

This will, in general, cause the operating crank 54 to settle with itscross piece 58 abutting the frame 41 at 59 with some clearance betweenthe projection 15 and'the cross piece 58. It is, however, possible thatthe final holding bias position j of the crank 54 may be reached at theend of the clockwise movement of the actuating lever 14.

r In this case, the projection 15 may continue to abut the operatingcrank cross piece 58. The

crossbar 13 may beactuated to effect the turning ment at a point 63 andto the frame at 64 and tend to turn the abutment clockwise about itspivot 52. The latch return spring 62 is connected to turn the latchclockwise about its pivot 482 The relative rotations of the abutment andits sustaining latch 41 are limited by an intercona necting link 65.which has one end pivoted to."

the abutment at 63 and-the other end pivoted to the latch 66 in a lostmotion slot of the link 65.

For quickly moving the contact l5 to the open! ing position and exertingpressure between the I contact surfaces l8 and I! at all times, thereare provided suitable energy storing means such as biasing springs 61,only oneof which appears and a rod 59 fixed in themovable' contact l5but of the actuating lever 14 to reset the circuit acting on a, rod 16pivoted at 11 on a crank 18 securedto the crossbar 13.

between the latch 41 and the frame as and tends]; 'Cmsdermg the mamconducting h through the primary relatively stationary contact r and themovable contact l5, it will be apparent that these two contacts are inseries in a current conductingloop which includes the terminal 13,

the stud H), the stud contact block 20, the por- I tion, of the movablecontact l5 between its engagement' with the contact block 20 and thecontact faces 28 of the contact I5, the contact fingers l5 and 16', the.contact block 23 of the stud H,

the stud II, and the terminal I4.

, Inasmuch as the curved contact surface l8- faces toward the inside ofthis loop, it will be v apparent that any magnetic loop expansion effect1 due to current flowing through the conducting extending throughslotted openings, not shown,

in the cradle sides 2| so as to permit a predetermined movement ofthe'contact I5 relatively to,

loop just traced will force the contact surface Y is of the contact l5against the contact surface because of the pendulous mounting of therelatively' stationary contact I 6, the magnetic loop expansion effecttends to force the lower end of this contactwith its contact surfaces 28against the contact surface 29 on the movable contact I 5 and, also, toforce the upper end of the contact l6 against the contact surface 24 inthe contact block 23. Thus, as long as the contact I5 is held by theoperating mechanism above described, the greater the current flowingthrough the loop, the greater the contact pressure between the contactsurfaces 28 and 29 and, also, between the upper end of the contactfingers l6, l6 and the contact surface 24 in the contact block 23. Inother words, the greater the current flowing through the loop of whichthe contacts l5 and I6 constitute a part, the greater the contactpressure at all the contacting portions as long as the mechanismmaintains the contacts in the closed position.

Considering now the arcing contact I! in relation to the movable contactl5, it will be observed that these contacts are in a conducting loopcomprising the terminal l3, the stud 10, the contact block 20, thecontact surfaces l8 and IS, the movable contact IS, the contact surfaces38, the contact II, the contact recess 33 in the arcing contact block34, the blowout coil 40, the stud II, and the terminal [4. Again, inconsequence of the pendulous suspension of the contact [I with contactbetween the surfaces 38 intermediate the axes of rotation of thecontacts l5 and H, the magnetic loop expansion effect consequent uponcurrent through the conducting loop just traced tends to increase thecontact pressure between the contact surfaces 38 and between the upperend of the contact I! and the contact recess 33. The greater the currentthrough the loop, the greater the contact pressure as long as thecontact is prevented from moving.

Since in case of an arc-back the other or sound anode circuits aresubjected to heavy current flow, it is obvious that circuit breakersembodying my invention will not blow open 'because'the increase in theloop expansion effect by virtue of the increase in current merely servesto intensify the contact pressure and thus provides better conductivity.Accordingly, circuit breakers embodying my invention are particularlysuitable for the protection of the anode circuits of mercury arcrectifiers because of this contactpressure feature as well as theaccelerated highspeed opening which is produced by the magnetic loopexpansion effect. It is to be understood, however, that the applicationof my invention is not limited solely to circuit breakers for theprotection of mercury arc rectifiers since it will be apparent to thoseskilled in the art that it can be used wherever desired and, inparticular, where circuit breakers have to carry currents in excess oftheir normal rated capacity for relatively short periods.

However, when for any reason the abutment 46 is released, as by virtueof the operation of electro-responsive means 60 to effect the release ofthe latch 41, the contact I5 is free to move to the circuit openposition shown in Fig. 2. Inasmuch as'each of the relatively stationarycontacts IE and H has a certain limited counter-clockwise movement aboutits contact axis, these contacts are free to follow the movable contactl5 momentarily and impart thereto by virtue of the magnetic loopexpansion effect an accelerated movement in the circuit openingdirection whereby to expedite the circuit opening operation. In thiscircuit opening operation, the sequence of breaking is first, betweenthe contact surface 28 of the contact finger l6 and the contact surface29 of the movable contact l5; second, between the contact surface 28 ofthe contact finger l6 and the contact surface 29 of the movable contactl5; and last, between the contact surfaces 38 of the arcing contact 11and the movable contact [5. Throughout all of this movement, it will beobserved that as long as any current flows through the loops includingeither the contact IE or the contact II, the magnetic loop expansioneffect continues to increase the contact pressure between the stationarycontact surface l8 and the contact surface 19 of the movable contact l5because the contact surface I 8 faces toward the inside of the loop insuch a manner that any tangential component of pressure tending to slidethe contact l5 off from the contact surface [8 is exertedon the pin 22which i constrained to move about the pivotal axis of the cradle 2!.Accordingly, this tangential component of pressure cannot effectrelative movement of the contact surfaces 18 and Hi. In other words, thepressure between the contact surfaces l8 and I9 is directed in such amanner as to maintain good contact at these surfaces without causing anysliding action or scuffing thereof in consequence of the loop expansioneffect.

While I have shown and described my invention in considerable detail, Ido not desire to be limited tothe exact arrangements shown, but seek tocover in the appended claims all those modifications that fall withinthe true spirit and scope of my invention.

What I claim'as new and desire to secure by Letters Patent of the UnitedStates is:

v 1. An electric switch comprising a plurality of relatively movablecooperating conducting members mounted to form a generally U-shapedcurrent conducting loop when the switch is closed, said members being soarranged that the magnetic expansion effect of current through the loopformed by the members tends to increase the pressure at the engagingportions thereof, said members comprising two spaced current conductingstuds, a first contact fulcrumed in one of aid studs for movementrelatively thereto, a contact surface on the other stud facing towardthe inside of the loop, a movable contact having a contact surfacecooperating with the contact surface on said other stud, and meanssupporting said movable contact for movement on the contact surface ofsaid other stud into engagement with said first contact intermediatesaid studs.

2; An electric switch having a generally U- shaped current conductingloop comprising two spaced current conducting studs, a first contactfulcrumed in one of said studs for movement relatively thereto, a convexcontact surface on the other stud facing toward the inside of the loop,a movable contact having a contact surface cooperating with said convexcontact surface, and movable means supporting said movable contact withthe cooperating contact surface thereon in rocking engagement with saidconvex contact surface, said movable contact engaging said first contactintermediate said studs upon actuation of saidmovable supporting meansin a direction to complete the loop.

3. An electric switch having a generally U-shaped current conductingloop comprising two spaced current conducting studs, a first contactfulcrumed in one of said studs for movement relatively thereto, acylindrical contact surface on the other stud facing toward the insideof the loop, a support pivotally mounted for movement about an axislying in said cylindrical surface, a contact pivotally mounted on saidsupport and having a plane contact surface in rocking engagement withsaid cylindrical contact surface, aid movable contact engaging saidfirst contact intermediate said studs upon movement of said support inone direction.

4. An electric switch comprising a stationary contact surface, apivotally supported contact having a contact surface in rockingengagement with said stationary contact surface, and a relativelystationary contact fulcrumed for movement about an axis and engageableby said pivotally supported contact between the axis of said relativelystationary contact and the engaging portions of said contact surfaces,the position of said axis and the positioning of said contact surfacesbeing such that the magnetic loop expansion effect consequent uponcurrent conducted by said contacts tends to increase the pressurebetweenthe engaging portions of said contacts and between said contactsurfaces and also to accelerate the circuit opening movement of thepivotally supported contact when this contact is free to move to thecircuit opening position.

5. An electricfswitch comprising a stationary curved contact surface, asupport pivoted for crease the pressure between the engaging portions ofsaid contacts and between said contact surfaces and also to acceleratethe circuit opening movement of the pivotally supported contact whenthis contact is free to move to the circuit opening position.

6. An electric switch comprisinga conductive stud having a substantiallycylindrical contact surface, a support pivoted for movement about agiven axis lying substantially in said cylindrical contact surface, amain contact pivotallymounted on said support and having a substantiallyplane contact surface in rocking engagement with said cylindricalcontact surface as said support is turned about its axis, and primaryand second-v ary contacts fulcrumed for. movement each about it's ownindependentaxisand respectively engageable by said main contact betweensaid given axis and the respective axes of the primary and secondarycontacts, the arrangement of said axes andithe positioning 'of saidcylindrical and plane contact surfaces being such that the magnetic loopexpansion effect consequent upon current conducted by said contactstends to increase the pressure between the main contact and the primaryand secondary contacts and between said plane and cylindrical contactsurfaces and also to accelerate the circuit opening movement of the maincontact when this contact is free to move to the circuit openingposition.

7. An electric switch having a generally U-shaped current conductingloop comprising two spaced current conducting studs, a flrstconmovementabout an axis tact fulcrumed in one of said studs for movementrelatively thereto, a contact surface on the other stud facing towardthe inside of the loop, and a movable contact having a contact surfacecooperating with the contact surface on said other stud, and a secondgenerally U-shaped current conducting loop comprising said studs, anarcing contact block mounted on but insulated from said one of saidstuds, an arcing contact fulcrumed in said arcing contact block formovement relatively thereto, and a current conducting member betweensaid arcing contact block and said one stud, and means supporting saidmovable contact for movement on the contact sur-- face of said otherstud into engagement sequentially with said first contact intermediatesaid studs and then with said arcing contact inter mediate said otherblock.

8. An electric switch having two generally U-shaped electricallyparallel current conducting loops, one of said loops comprising twospaced current conducting studs, one oil-which includes a contact block,a first contact fulcrumed in said contact block for movement relativelythereto, a convex contact surface on the other stud facing toward theinside of said two loops, a movable contact having a contact surfacecooperating with said convex contact surface, and the other of saidloops comprising an arcing contact block mounted on said one contactblock, a conductive connection between said arcing contact block andsaid one stud, an arcing contact fulcrumed in said arcing contact blockfor movement relatively thereto, and movable. means supporting saidmovable contact with the cooperating contact surface thereon in rockingengagement with said convex contact surface, said movable contactengaging said arcing contact and said first contact in apredeterminedsequence upon actuation of said movable supporting .meansin a direction to complete said loops.

9..A circuit breaker comprising two spaced current conducting studs, tworelatively stationary contacts respectively mounted in currentconducting relation with one of said studs to turn about spacedpivotalaxis and biased in one direction for a limited turningmovement, a maincontact mounted for turning in the opposite direction into currentconducting-engagement with both of said relatively stationary contactsto complete electrically parallel current conducting loops through saidstuds, the mounting for said main contact comprising a substantiallycylindrical contact surface on the other of said studs facing toward theinside. of said loops, a support pivoted for movement about an elementof said a cylindrical contact surface, and'a pivot on said support forsaid main contact arranged to permit rocking movement thereof on saidcylindrical surface upon actuation of said support, the arrangementbeing such that the magneticlloop expansion effect consequent uponcurrent conducted by the three contactsiin the electrically the circuitopening movement' of the main contact when it is free to move to thecircuit opening position. I

10. An electric switch comprising upper main and arcing contact blocksand a lower contact block, means for conducting current between saidstud and said arcing contact lower contact block and said upper main andarcing contact blocks comprising two relatively low mass movablecontacts respectively in contacting engagement with said upper main andarcing contact blocks, a movable contact of relatively low mass incontact engagement with said lower contact block movable into engagementto complete a conducting path between said lower contact block and saidmain and arcing contact blocks first to engage the movable contact incontacting engagement with the arcing contact block and then to engagethe movable contact in contacting engagement with the main contactblock, said movable contacts being so constructed and arranged thatwhile the witch is closed, the magnetic loop expansion effect due tocurrent flow through the contacts tends to increase the contact pressureat their contact engaging portions and to facilitate current transferfrom the main contact block to the arcing contact block during thecircuit opening movement of the contacts.

11. An electric switch comprising a plurality of relatively movablecooperating conducting members mounted to form a generally U-shapedcurrent conducting loop when the switch is closed, said members being soarranged that the magnetic expansion effect of current through the loopformed by the members tends to increase the pressure at the engagingportions thereof, said members comprising two spaced currentconductingstuds, a contact surface on each stud facing toward the inside of theloop, and movable cooperating contacts respectively fulcrumed on saidcontact surfaces.

12. An electric switch comprising a plurality of relatively movablecooperating conducting members mounted to form a generally U-shapedcurrent conducting loop when the switch is closed, said members being soarranged that the magnetic expansion effect of current through the loopformed by the members tends to increase the pressure at the engagingportions thereof, said members-comprising two spaced current-conductingstuds, a contact surface on each stud facing toward the inside of theloop, and contacts respectively fulcrumed on said contact surfaces andengageable with each other intermediate said studs to complete aconducting path therebetween.

13. An electric switch having a generally U-shaped current-conductingloop comprising two spaced current-conducting studs, one of said studshaving a cylindrical contact surface concave toward the inside of saidloop and the other of said studs having a cylindrical contact surfaceconvex toward the inside of the loop, and movable cooperating contactsrespectively mounted for movement on said cylindrical contact surfaces.

14. An electric switch having a generally U-shaped current-conductingloop comprising two spaced current-conducting studs, one of said studshaving a cylindrical contact surface concave toward the inside of saidloop and the other of said studs having a cylindrical contact surfaceconvex toward the inside of the loop, and a plurality of contactsrespectively fulcrumed on said cylindrical contact surfaces andengageable intermediate said studs to complete a conducting paththerebetween.

' JOSEPH W. SEAMAN.

